This invention relates to fiber-reinforced asphalt-type overlay compositions capable of providing surfaces of improved durability, and to a method for obtaining such durability while maintaining acceptable lay down and workability characteristics.
A substantial number of existing highways are paved or faced with asphalt-type compositions containing various aggregates such as gravel, crushed stone, sand and the like. Moreover, additional miles of road are built each year in which asphalt-type compositions serve as a major component. The continued widespread use of such material, however, depends, to a great extent, upon initial cost, durability, and the amount and cost of maintenance.
Asphalt-type compositions of the more recent types frequently contain fiber reinforcing component(s) such as various concentrations of polyolefin-type fiber(s) within asphalt, asphalt/rubber, rubber, or acrylic copolymer-asphalt/acrylic copolymer-type compositions, and the like, which are compatible with existing lay-down and one or more working steps (i.e. spreading, rolling and curing) techniques and equipment.
In general, when surfacing or overlay steps are being carried out, either hot mix or emulsified asphalt-type compositions are applied respectively (1) as filler for underlying cracks, (2) as a waterproof layer between old and new paving surfaces, and (3) as an external surfacing material. In order to serve such needs, however, the amount and fineness of aggregate, the concentration and/or presence of a fiber reinforcing component, and the use of various art-recognized additives such as blending and curing agents etc. (ref. U.S. Pat. No. 4,492,781) can correspondingly vary.
In general, however, overlay compositions suitable for the above-indicated purposes must be capable of providing a high degree of durability, flexibility and shear resistance. It is also very useful and desirable, as a commercial goal, that overlay compositions be applicable within as wide a range of temperature conditions as possible, and be easily, worked.
It is commonly accepted that durability and shear resistant properties can be improved, under existing technology, by the inclusion of relatively small amounts of reinforcing fiber staple component such as wire, fiberglass, asbestos, and particularly synthetic polymeric material within the overlay composition.
Particularly preferred synthetic staple fibers for such purpose, are polyolefin fibers such as polypropylene and polyethylene fiber, because of their general compatibility with asphalt and asphalt-type compositions. Such reinforcing fiber components are generally added as short staple and customarily vary in concentration from about 0.10 to about 10 weight percent, and preferably in an amount of about 0.25-8.0 weight percent based on solids.
Length-wise, such "short staple" reinforcing fiber component(s) normally vary from about 1/4" to about 3/4", depending upon the intended purpose of the overlay and the method of application.
For example, when high-performance surfaces such as highways, airfields, tennis courts and the like are being repaired, it is advantageous to obtain-as high a degree of fiber comingling and interlocking as possible, the practical upper limit being about 3/4" fiber length and within a mid range of the above-indicated conventional concentrations. The addition of longer staple or higher concentrations, particularly synthetic hydrophobic fiber material such as polypropylene, usually results in adverse pumpability and fiber clumping. Moreover, fiber-reinforced asphalt-type overlay compositions, as above described customarily require at least a 10.degree. C. higher temperature than the usual 284.degree. F.-310.degree. F. (140.degree.-155.degree. C.) range, in order to maintain a workable lay-down viscosity or temperature window in the field. Such increased temperature is conventionally difficult to control (i.e. hot spots) and likely to result in at least some degradation of conventional monocomponent hydrophobic reinforcing fibers.
It is an object of the present invention to improve the structural stability and durability of asphalt-type overlays formed using asphalt-type overlay compositions, while continuing to enjoy the benefits of hydrophobic polyolefin-type reinforcing fiber and without resorting to increased staple length or higher-than-normal concentrations of the fiber.
It is a further object of the present invention to achieve such improvement without creating additional problems with respect to lay down and workability of fiber-reinforced overlay compositions.
It is still a further object of the present invention to avoid fiber clumping and increase structural stability of asphalt-, asphalt/rubber-, rubber-, acrylic copolymer-, and asphalt/acrylic copolymer-containing overlay composition surfaces and to permit more effective use of reinforcing fiber material, particularly polymeric polyolefin-type staple fiber such as polypropylene fiber.